Beam splitting prisms

ABSTRACT

An optical colour separating arrangement comprising a first plane colour-selective dichroic semi-reflector inclined at an angle of incidence Alpha to the axis of the beam incident on said semi-reflector, Alpha being less than 30*, a second plane colour-selective dichroic semi-reflector inclined at an angle of incidence Beta to the axis of the beam transmitted through the first semi-reflector, Beta lying between 20* and 50* and being greater than Alpha , a single plane total-reflector receiving the reflected beam from the second semi-reflector 0and directing said beam along an axis at least approximately parallel to the axis of the transmitted beam, and at least one total reflector for the beam reflected from the first semi-reflector, the arrangement being such that all the colour constituents undergo either zero or an even number of reflections.

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United States Patent (1 1 [111 3,802,763 Cook et al. 1 Apr. 9, 1974 1 BEAM SPLITTING PRISMS 3.534.158 10/1970 Eilenberger 350/173 {75] Inventors: Gordon Henry Cook; John Anthony I l F awcett; Gordon Whitehead, all of Prlmary bmmmep'Dlavld Leicester England Attorney, Agent, or FzrmBrisebois & Kruger 731 Assignee: The Rank Organisation Limited? London, England [57] ABSTRACT [22] Filed: 7, 1972 An optical colour separating arrangement comprising a first plane colour-selective dlCl'll'OlC' semi-reflector PP Nod ,216 inclined at an angle of incidence a to the axis of the beam incident on said semi-reflector, a being less than 30 F A P 1 D t 30, a second plane colour-selective dichroic semi- 1 I Ts; y y a a 40756 7 I reflector inclined at an angle of incidence B to the axis Sept mat mam of the beam transmitted through the first semireflector, B lying between 20 and 50 and being l (5 IIIIIIIIIIIIIIIIIIIIIII 65 greater than a, a single plane total-reflector receiving 58 d 178/5 4 E the reflected beam from the second semi-reflector 1 0 care Oand directing said beam along an axis at least approximately parallel to the axis of the transmitted beam. [56] References Cited and at least one total reflector for the beam reflected UNITED STATES PATENTS from the first semi-reflector, the arrangement being 3,202,039 8/1965 De Lang et a1. .1 350/173 such that all the colour constituents undergo either 3.511460 6/]970 Baluteau 6! al 350/]73 ero or an even number of reflections 3,602,637 8/l97l Katsuta et al..... 350/173 X 3.610.818 10/1971 Bachmann 350/173 X 7 Claims, 4 Drawing Figures PATENTEDAPR 9 I974 3302.763

saw u or 4 BEAM SPLITTXNG PRISMS This invention relates generally to a method of splitting a beam of light into three components, especially colour constituents, and to a beam splitting and colour separating arrangement for carrying out said method, and more particularly to a beam splitting prism more especially for use in combination with an optical objective in a colour television camera optical system.

In a colour television camera, it is necessary to split the beam of light received through the objective into three components respectively having different spectral characteristics. The three images associated with these components are received by three imagereceiving tubes, which are of appreciable axial length, and therefore predominantly determine the overall dimensions of the camera. Compactness of layout is assisted if two of the image receptors are parallel to one another.

When, as is usual, the three image planes have different angular relationships with respect to one another, the necessarily long receiving tubes are disposed in different directions, impairing the compactness of the overall arrangement. Since in preferred arrangements there is insufficient axial space to introduce further refleeting surfaces solely for the purpose of re-orientating one or more image planes to lie in the same or similar angular relationship to another, one or more primary images must be relayed to a secondary image by an optical system within which the required angular change can be incorporated. Sometimes there is a preference to use the blue component for this purpose, particularly when it is desirable to incorporate an optical reduction in format size to adjust the relative image brightness in a manner which reduces the lag characteristics when viewing movements in the object space. This preference may arise in presence of studio incandescent illumination, deficient in blue light.

One beam splitting arrangement for a colour television camera is known from British Specification No. 983933. In this known arrangement, the incident beam is successively incident on two dichroic layers inclined in opposite senses to the axis of the incident beam, in each case at angles of 25 55 and 13 respectively. The first dichroic layer is carried on the rear surface of a wedge prism having a front polished surface normal to the incident beam. Spaced behind the first dichroic layer by a small air gap is the front polished surface of a triangular prism having a rear surface carrying the second dichroic layer. Behind the second dichroic layer is the front surface of a second wedge prism having its rear polished surface normal to the incident axis. The portion of the beam transmitted through the two dichroic layers, the green constituent, emerges through this rear surface of the second wedge prism. The beam reflected at the first dichroic layer, the blue constituent, is internally reflected at the front polished surface of the first wedge prism and emerges normally through a lateral polished surface of said prism at an acute angle, about Sl degrees, to the axis of the transmitted beam. The beam reflected at the second dichroic layer, the red constituent, is reflected at the air gap behind the first dichroic layer to emerge normally through the third surface of the triangular prism at a larger acute angle, about 78 degrees, to the axis of the transmitted beam.

The arrangement has the advantage that all three emergent beams have suffered zero or an even number of reflections, which means that the three images are optically handed vertically and left-to-right in the same sense. This facilitates the problem of achieving registration of the colour signals to be generated, because provided the geometry of electron beam scanning in the three image receptors is identical there is no need for such geometry of scanning to be perfect in itself. This remains true even when a relay lens is introduced into the blue channel in order to give a reduced image format. The blue beam is inverted by such relay lens both vertically and horizontally, and this can be compensated for by rotating the blue image receptor and its deflection coils through 180 about its optical axis.

However, the known arrangement is very disadvantageous with regard to compactness of layout. In order to bring two of the emergent beams into parallel relationship, it is known to bring the blue beam out of the lateral surface of the first prism into a fourth prism which has a plane polished surface at which the blue beam is internally reflected along an axis parallel to the axis of the transmitted beam, said fourth prism having another polished surface through which the blue beam emerges normally. This creates the difficulty, however, that the third reflection introduces an inversion into the blue beam in one direction only. As a result, to solve the resultant registration problem, the geometry of scanning in the image reflectors must be made geometrically perfect, which is difficult and expensive to achieve.

The last-mentioned difficulty can be overcome by replacing the additional plane reflector for the blue beam by a compound reflector of the roof-prism type. Unfortunately, a difficulty of similar severity then arises. Not only is a roof prism difficult and expensive to manufacture, but also it occupies a longer path length than the corresponding plane reflector. The distance between the primary image and the relay lens has to be increased to accommodate the roof prism so that for a given reduction factor, the focal length of the relay lens has to be increased, further increasing the distance between the primary and secondary images, and adding significantly to the problem of aberration correction in design of the lens. This detracts appreciably from the compactness of layout which is being sought.

The present invention has for its general object to provide an improved method and arrangement for splitting a beam of light into three colour constituents, and for a more specific object, to provide a colour separating arrangement for a colour television camera which enables a compact layout to be achieved without incurring difficulties as to image registration.

In one aspect, the present invention provides a method of splitting a beam of light into three components, especially colour' constituents, according to which the beam is successively incident on two plane semi-reflectors, especially colour-selective dichroic semi-reflectors, with an angle of incidence at the first semi-reflector of less than 30 and an angle of incidence at the second semi-reflector lying between 20 and and greater than the angle of incidence at the first semi-reflector, where the angle of incidence is defined as that angle between the optical axis and the normal to the semi-reflector, and the reflected beam from the second semi-reflector is then incident on a single plane surface affording total reflection, preferably to cause said second reflected beam to be directed along an axis at least approximately parallel to the axis of the transmitted beam, the method being such that all the colour constituents undergo either zero or an even number of reflections. The most important respect in which this method differs from the known arrangement is that the angle of incidence at the second dichoroic semi-reflector is increased. In practice, as will be explained later, it is this feature that enables the beam reflected from this second semi-reflector then to be totally reflected by a single plane reflector along an axis parallel to that of the transmitted beam. Further, for application to a colour television camera of high quality, especially with regard to colorimetry, the method is only practical when the first dichroic semi-reflector is adapted to give peak reflection at a wavelength spectrally positioned between the wavelengths of peak reflection and peak transmission at the second dichroic semi-reflector. This will mean that the first reflected beam is a green constituent. The transmitted beam is preferably a red constituent and the second reflected beam a blue constituent.

Preferably, the reflected beam from the first semireflector is subsequently incident on a single plane surface affording total reflection to cause said first reflected beam to be directed along an axis at an acute angle to the axis of the transmitted beam. Thus, one of the emergent beams undergoes zero reflections and the other two emergent beams each undergo two reflections. Difficulties over image registration are thus avoided, and the introduction of a relay lens into the blue channel to reduce the blue image format can be catered for, as with the known arrangement, by rotation through 180 degrees of the blue channel image receptor.

The three emergent beams may be contained in a single plane or, alternatively, the first reflected beam, the green channel, may be brought out by means of a single plane surface affording total reflection so that the axes of the first reflected beam and the transmitted beam lie in a plane normal to the plane containing the axes of the second reflected beam and the transmitted beam.

in another aspect, the present invention provides an optical colour separating arrangment comprising a first plane colounselective dichroic semi-reflector inclined at an angle of incidence a to the axis of the beam incident on said semi-reflector, a being less than 30, a second plane colour-selective dichroic semi-reflector inclined at an angle of incidence B to the axis of the beam transmitted through the first semi-reflector, B lying between and 50 and being greater than a, a single plane surface affording total reflection receiving the reflected beam from the second semi-reflector, and preferably directing said beam along an axis at least approximately parallel to the axis of the transmitted beam, and at least one surface affording total reflection for the beam reflected from the first semi-reflector, the arrangement being such that all the colour constituents undergo either zero or an even number of reflections. The arrangement preferably includes a single plane surface affording total reflection for the reflected beam from the first semi-reflector and directing said beam along an axis inclined at an acute angle to the axis of the transmitted beam. B will usually be not less than and not more than 45.

A practical construction for the arrangement comprises a first prism element having a first polished surface on which the beam of light is first incident at an angle of incidence A, A being less than 40, and having a second polished surface associated with the first semireflector at the angle a to the incident axis, and a second prism element behind the first element having a first polished surface associated with the first semireflector at the angle a to the incident axis and having a second polished surface associated with the second semi-reflector at the angle B to the incident axis. Conveniently, in this construction, the beam reflected at the first semi-reflector is subsequently totally reflected internally at the first polished surface of said element, said first prism element having a third polished surface normal to the axis of said totally reflected beam and through which said beam is emergent. A third prism element is preferably provided behind the second element, this third prism element having a first polished surface associated with the second semi-reflector at the angle B to the incident axis, and a second polished surface normal to the incident axis through which the transmitted beam is emergent.

In one embodiment, the second prism element has a third polished surface at which the beam reflected at the second semi-reflector is reflected along an emer gent axis substantially parallel to the axis of the incident beam through a fourth polished surface on said element which lies normal to said emergent axis. In this embodiment, )t is conveniently equal to zero.

In a more useful embodiment particularly suitable for application in a colour television camera, the second prism element carries an auxiliary prism element into which the beam reflected from the second semireflector enters normally through a plane contact surface, said auxiliary element having a first polished surface at which said beam is totally reflected along an emergent axis through a second polished surface on said auxiliary element which lies normal to said emergent axis. This embodiment conveniently includes an entrance prism element in front of the first prism element having a first polished surface normal to the axis of the incoming beam of light and a second polished surface spaced by an air gap in front of the first polished surface of the first element at the angle A to the incident beam axis. The small air gap behind the entrance prism enables the angle of incidence on the first dichroic semi-reflector to be reduced whilst still achieving total internal reflection at this air gap of the beam reflected from the first semi-reflector.

In a colour television camera, the colour separating arrangment receives the beam of light to be split from the camera objective. ln this instance, or in other applications where the colour separating arrangement is operating in conjunction with an optical objective, the pa rameters of the colour separating arrangement need to be related to the relative aperture of f-number of the objective. Preferably therefore, in such a combination:

where f is the f-number of the objective and n is the mean refractive index of the material of which the "prism elements are made, all said elements being made of the same material. Within these broad limits, more specific relationships will usually be appropriate, according to which:

where fis the f-number of the objective and n is the mean refractive index of the material of which the prism elements are made, all said elements being made of the same material.

In a third aspect, the present invention provides a colour television camera optical system comprising an optical objective and a colour separating prism arrangement, said colour separating arrangement comprising a first prism element having a front polished surface normal to the incident optical axis and a second polished surface inclined to the incident axis by an angle A; a second prism element separated from the first by a thin parallel air gap so that its first polished surface is also inclined to the incident axis by the angle A, a second polished surface of said second element carrying or being closely adjacent to a first colour selective partially reflecting surface which is inclined to the incident axis by an angle a; a third prism element cemented to or closely adjacent to the second prism element so its first polished surface is inclined to the incident axis by the angle a, a second polished surface of said third element carrying or being closely adjacent to a second colour selective partially reflecting surface which is inclined to the incident axis by an angle B; a fourth prism element cemented to or closely adjacent to the third prism element so that its first polished surface is inclined to the incident axis by the angle B, said element having a second polished surface normal to the incident optical axis; and wherein the second prism element has a third polished surface normal to the optical axis of the beam reflected by its second and first surfaces, whilst the third prism element either carries a total reflector for the beam reflected from the second partial reflector or cooperates with a further prism element carrying such total reflector; the beam emerging through a polished surface normal to the optical axis of the beam reflected by the second partial reflector and said total reflector, the arrangement being such that: [Sin (l/n) Sin (l/2nf) )t [Sin (l/n) Sin (l/2nf) 2a where f is the f number or relative aperture of the camera objective, and n is the refractive index of the prism glass material,

When a high standard of colorimetry is essential, the more specific relationships previously stated will usually be adhered to. These more specific relationships will invariably apply when, as mentioned earlier, the first dichroic semi-reflector is adapted to give peak reflection at a wavelength spectrally positioned between the wavelengths of peak reflection and peak transmission at the second dichroic semi-reflector Practical embodiments of colour separating arrangement in accordance with the invention will now be described by way of example, more particularly with reference to their application in a colour television camera, and referring to the accompanying drawings, in which:

FIG. 1 shows a preferred embodiment when viewed in a direction normal to the plane containing the optical axes of the incident beam and of the split constituent beams, which are all coplanar;

FIG. 2 shows an alternative and simpler arrangement suitable for less exacting applications, from a viewpoint corresponding to that of FIG. I;

FIG. 3a shows a modification of the embodiment of FIG. 1 from a similar viewpoint, in which part of the arrangement is turned through 90 degrees about the axis of the incident beam, so that a particular one of the split constituent beams emerges towards the viewpoint; and

FIG. 3b shows the modification of FIG. 3a viewed in a direction normal to the plane containing the axes of the incident beam and the said particular one split constituent beam.

In a first embodiment shown in FIG. 1, the beam of light to be split is incident normally on the front polished surface 1a of an entrance prism l in the form of a wedge, which has a rear polished surface lb inclined at an angle A to the incident beam, A being equal to 24. Spaced by a small air gap behind the rear surface of the entrance prism is the front polished surface 2a, also inclined to the optical axis at the angle A, of a first main prism 2 also of wedge form which on its rear surface 2b carries a first plane dichroic semi-reflector inclined at an angle a to the optical axis, in the opposite sense to the inclination A, a being equal to 15. Adjoining the first semi-reflector is the front polished surface 3a of a second main prism 3, of quadrilateral form, which has a rear surface 3b carrying a second plane dichroic semireflector inclined at an angle B to the optical axis, in the opposite sense to the inclination a, B being equal to 335. Behind the second main prism is a third main prism 4 having a front polished surface 4a adjoining the second and a rear polished surface 40 normal to the incident axis.

The first dichroic semi-reflector 2b is adapted to reflect the green constituent G of the beam, and transmit the blue and red constituents to the second dichroic semi-reflector, where the blue constituent B is reflected. The red constituent R is transmitted along the optical axis to emerge normally through the rear surface of the third main prism to be received directly by the image receptor R, for the red channel.

The green constituent G reflected at the first semireflector 2b is totally reflected internally at the air gap lb, 2a behind the entrance prism I, to emerge from the first main prism 2 along an axis inclined at about to the optical axis, passing normally through a polished lateral surface 2c on the first main prism to the image receptor Gr for the green channel.

The blue constituent B reflected at the second semireflector 3b emerges directly from the second main prism 3, passing normally through alateral surface 3c thereof cemented to a first polished surface 50 of a supplementary prism 5 adjoining and carried by said second main prism. The supplementary prism 5 has a second polished surface 5b at which the blue constituent is totally reflected along an axis parallel to the incident axis, passing normally through a third polished surface 50 on said supplementary prism, through a relay lens 8, for reducing the image format of the blue image, to the image receptor B, for the blue channel. Thus, with this arrangement, the image receptors R, and B, for the red and blue channels lie in parallel relationship.

The arrangement is especially intended for use in a colour television camera in combination with an optical objective, )]it receives the light beam to be split, having a relative aperture f/l.6. The prisms are all made of a glass having a refractive index 2a= 1.518. The advantages of the arrangement can best be understood by making a comparison with the known arrangement previously described. The fundamental difference is the increase in the angle of incidence at the second semi-reflector. This increase in angle enables the blue constituent to be brought directly out of the second main prism and reflected at a single plane reflector back along an axis parallel to the incident and transmitted beam. In other words, reflection of the beam at an air gap behind the first dichroic semi-reflector is avoided. However, for utilisation in an efficient colour television optical system, which has relatively stringent colorimetry requirements, two further differences from the known arrangement are of prime importance.

Firstly, the entrance prism is introduced to provide a plane air gap at the angle A to the optical axis. The angle of incidence at the first semi-reflector can thus be reduced whilst still achieving total internal reflection, at said air gap, to cause the green constituent to emerge directly from the first main prism. ln this connection, it will be understood that the spectral characteristics of dichroic layers vary with the angle of incidence and, generally speaking, disadvantageous effects with regard to colorimetry become more pronounced when the angle of incidence is increased. A reduction in this angle not only improves the quality of the beam in the reflected green channel, but also those of the beams transmitted to the blue and red channels.

Secondly, the order of split of the incident beam is changed from blue-red-green, as in the known arrangement, to green-blue-red. Again, it is well known that the spectral response of a dichroic semi-reflector varies with angle of incidence and polarisation of the incident beam. adverse effects are concentrated around the cutoff edge, and become more pronounced with an increasing angle of incidence In the known arrangement, the dichroic semireflectors are inevitably working near the cut-off edges which separate one channel from another, and it is therefore difficult to achieve high colorimetric standards when the angle of incidence at the semi-reflector exceeds degrees.

However, in the present arrangement, having taken off the green part of the spectrum first, it is possible to locate the cut-off edge of the second dichroic semireflector separating the red and blue constituents in the green part of the spectrum. Thus, increased adverse effects around the cut-off edge do not affect the red and blue channels. This fact taken in conjunction with the greater freedom afforded by the reduced angle of incidence at the first semi-reflector enables the angle of incidence at the second semi-reflector to be considerably increased, as previously described. Even with the increased angle of incidence at the second semi-reflector, the overall colorimetry characteristics of the present arrangement are an improvement on the known arrangement. The required spectral shape for the green channel can be reflected directly, with only a simple filter for secondary trimming and to absorb some unwanted out of band responses. Having extracted the green component, the red and blue channels are not shaped by the second dichroic semi-reflector, but by trimming filters acting in combination with the responses of the camera objective, relay lens and image receptors. The amplitude in the green channel is reduced, as is known to be desirable, possibly without the use of a neutral density filter. A further advantage is that both dichroic semi-reflectors are located between glass, and can be cemented between glass, .giving greater spectral stability and less risk of contamination than an air-backed dichroic layer.

In fact, the advantages of the above-described arrangement, relative to the known system, are most readily apparent when, as in practical application to a colour television camera, the effects of polarised light and/or light from off-axis object points are considered.

In an optimised form of the present arrangment in which the various parameters are selected for best overall performance an average error for twenty six test colours, which can conveniently be used to assess colorimetric fidelity, of less than 1.25 j.n.d. units is achievable where a j.m.d. unit" is a unit representing the magnitude of the average colour step which produces a "just noticeable difference to the human eye), for unpolarised light from an axial object point. Optimisation of the known system can only achieve a substantially comparable result in unpolarised light and on axis. However, considering an off-axis point, for example at the image corner of a typical image format, the optimised form of the present arrangement, for the same test colours, gives rise to an average error of about 2.5 j.n.d. units, compared with about 5.5 j.n.d. units for the known system. For polarised light from an axial object point, the comparative values are 2.5 j.n.d. units with the arrangement according to the present invention and 3.5 j.n.d. units for the known system. For off axis object points and polarised light these differences are appreciably larger.

In general, the variation in colour rendering over the image (colour shading) can be to some extent negated by electronic shading correction circuitry. Similarly, the effects of polarised light may be reduced by an optical retardation plate. However, in addition to their cost and added complexity, the provision of these features can impair the overall camera performance in other respects, such as signal to noise ratios and optical quality. The significance of the above comparative figures is that with the present arrangement the need for such measures is greatly reduced or even eliminated.

In association with the above, both signal strength and spectral shape tend to change appreciably with variation in field angle in the known arrangement, the former effect is substantially avoided and the latter effect greatly reduced in the above-described arrangement in accordance with the invention.

It can readily be determined that the above-described arrangement complies with the relationships previously mentioned:

[Sin (1/n)- Sin" (l/2nf)] A [Sin (l/n) Sin (1/2nf) 20:], l) [Sin (l/n)]/2 a Sin (l/2nf), 2) Sin (l/n) B a. (3)

FIG. 2 shows a second embodiment wherein corresponding references are used to denote elements and surfaces corresponding to similar elements and surfaces in the embodiment of FIG. 1.

In this second embodiment applicable where less high standards of colorimetry are necessary, the entrace prism, and the supplemenetary prism carried by the second main prism are dispensed with. The angle A thus becomes equal to zero. The angle a is equal to 25.5" and the angle B is equal to 45. The refractive index of the prism glass is 1.518 and the arrangement is adapted for use with an optical objective of relative aperture f/2.2. It can thus be ascertained that the arrangement complies with the relationships previously mentioned:

[Sin (l/n) Sin (l/2nf) A [Sin' (l/n) Sin (l/Znf) a 2) [Sin' (l/n) Sin (I/Znf) B a. (3)

In this second embodiment, the incident beam enters the first main prism 2 directly, through a front polished surface 2a normal to the optical axis. The green constituent G reflected at the first semi-reflector 2b is totally reflected internally at said front polished surface 2a to emerge directly from the first prism 2. The blue constituent B is reflected at the second semi-reflector 3b at right angles to the incident axis, to be internally reflected at a lateral polished surface 30 formed directly on the second main prism, thereby to emerge normally through a rear polished surface 3d on said second prism 3 to pass through a relay lens B, to the blue channel image receptor 8,. As before, this receptor B lies parallel to the red channel image receptor R,, which is aligned with the incident and transmission axis. Further relaxation of colorimetric requirements may make this second embodiment useful without changing the order of colour split from that used in the known arrangement.

In the above-described arrangements, the colour constituents are brought out along axes lying in a common plane. However, in a useful modification shown in FIGS. 3a and 3b, the green constituent is brought out on an axis lying normal to the plane containing the red and blue channel axes. In this modification, as applied to the first arrangement above-described, the entrance prism and the first main prism, together with the first dichroic semi-reflector and the front surface of the second main prism, are turned through 90 about the incident axis relative to the remainder of the system. The arrangement of FIGS. 3a and 3b will be clear without further description from a study of the references employed, which correspond with those used in FIG. 1.

We claim:

1. An optical color separating prism system comprising a first prism element having a first polished surface on which the beam of light to be split into color components is incident at an angle of incidence A, A being less than 40, and having a second polished surface inclined at an angle a to the incident axis, a being less than 30, a second prism element having a first polished surface cemented to the second polished surface of the first element and having a second polished surface inclined at an angle B to the incident axis, B lying between 20 and 50, a first dichroic layer contained between the cemented surfaces of the first and second prism elements, the color component reflected from said first dichroic layer being totally reflected at the first polished surface of the first prism element to emerge from said element through a third polished surface thereon along an axis inclined at an acute angle to the incident axis, a third prism element having a first polished surface cemented to the second polished surface of the second prism element and having said second polished surface normal to the incident axis, a second dichroic layer contained between the cemented surfaces of the second and third prism elements, the color component transmitted by both the first and second dichroic layers emerging along the incident axis through the second polished surface of the third prism element, and the second prism element having a third polished surface at which the color component reflected from the second dichroic layer is totally reflected along an axis substantially parallel to the incident axis to emerge normally through a fourth polished surface on the second prism element.

2. An optical system according to claim 1, wherein the second prism element comprises a basic prism unit carrying the first and second polished surfaces and an auxiliary prism unit carrying the third and fourth polished surfaces of said second prism element, the two units being cemented at a plane interface normal to the axis of the color component reflected from the second dichroic layer.

3. An arrangement according to claim 1, wherein B is not less than 30 and not greater than 45.

4. An arrangement according to claim I, in which A is equal to zero.

5. An arrangement according to claim 1, including an entrance prism element in front of the first prism element having a first polished surface normal to the axis of the incoming beam of light and a second polished surface spaced by an air gap in front of the first polished surface of the first element at the angle A to the incident beam axis.

6. A colour television camera optical system comprising an optical objective and a colour separating prism arrangement, said colour separating arrangement comprising a first prism element having a front polished surface normal to the incident optical axis and a second polished surface inclined to the incident axis by an angle A; a second prism element separated from the first by a thin parallel air gap so that its first polished surface is also inclined to the incident axis by the angle A, a second polished surface of said second element closely adjacent to a first colour selective partially reflecting surface which is inclined to the incident axis by an angle a; a third prism element closely adjacent to the second prism element so its first polished surface is inclined to the incident axis by the angle a, a second polished surface of said third element closely adjacent to a second colour selective partially reflecting surface which is inclined to the incident axis by an angle B; a fourth prism element closely adjacent to the third prism element so that its first polished surface is inclined to the incident axis by the angle B, said element having a second polished surface normal to the incident optical axis; and wherein the second prism element has a third polished surface normal to the optical axis of the beam reflected by its second and first surfaces, whilst the third prism element cooperates with a total reflector for the beam reflected from the second partial reflector; the beam emerging through a polished surface normal to the optical axis of the beam reflected by the second partial reflector and said total reflector, the arrangement being such that:

11 12 [Sin (l/n) Sin (l/2nfj] A [SirF (l/n) claimed in claim 6, in which n (l/n) Sin (l/2nf) 1 [Shr' (l/n) Sin (l/n) Sin (l/2nf) B sm (1/2"!) 200] where fis the fnumber or relative aperture of the camera objective, and n is the refractive index of the prism (Um/2 a (1/24,), (2)

glass material. Sin (l/n) B a. 3)

7. A colour television camera optical system as 

1. An optical color separating prism system comprising a first prism element having a first polished surface on which the beam of light to be split into color components is incident at an angle of incidence lambda being less than 40*, and having a second polished surface inclined at an angle Alpha to the incident axis, Alpha being less than 30*, a second prism element having a first polished surface cemented to the second polished surface of the first element and having a second polished surface inclined at an angle Beta to the incident axis, Beta lying between 20* and 50*, a first dichroic layer contained between the cemented surfaces of the first and second prism elements, the color component reflected from said first dichroic layer being totally reflected at the first polished surface of the first prism element to emerge from said element through a third polished surface thereon along an axis inclined at an acute angle to the incident axis, a third prism element having a first polished surface cemented to the second polished surface of the second prism element and having said second polished surface normal to the incident axis, a second dichroic layer contained between the cemented surfaces of the second and third prism elements, the color component transmitted by both the first and second dichroic layers emerging along the incident axis through the second polished surface of the third prism element, and the second prism element having a third polished surface at which the color component reflected from the second dichroic layer is totally reflected along an axis substantially parallel to the incident axis to emerge normally through a fourth polished surface on the second prism element.
 2. An optical system according to claim 1, wherein the second prism element comprises a basic prism unit carrying the first and second polished surfaces and an auxiliary prism unit carrying the third and fourth polished surfaces of said second prism element, the two units being cemented at a plane interface normal to the axis of the color component reflected from the second dichroic layer.
 3. An arrangeMent according to claim 1, wherein Beta is not less than 30* and not greater than 45*.
 4. An arrangement according to claim 1, in which lambda is equal to zero.
 5. An arrangement according to claim 1, including an entrance prism element in front of the first prism element having a first polished surface normal to the axis of the incoming beam of light and a second polished surface spaced by an air gap in front of the first polished surface of the first element at the angle lambda to the incident beam axis.
 6. A colour television camera optical system comprising an optical objective and a colour separating prism arrangement, said colour separating arrangement comprising a first prism element having a front polished surface normal to the incident optical axis and a second polished surface inclined to the incident axis by an angle lambda ; a second prism element separated from the first by a thin parallel air gap so that its first polished surface is also inclined to the incident axis by the angle lambda , a second polished surface of said second element closely adjacent to a first colour selective partially reflecting surface which is inclined to the incident axis by an angle Alpha ; a third prism element closely adjacent to the second prism element so its first polished surface is inclined to the incident axis by the angle Alpha , a second polished surface of said third element closely adjacent to a second colour selective partially reflecting surface which is inclined to the incident axis by an angle Beta ; a fourth prism element closely adjacent to the third prism element so that its first polished surface is inclined to the incident axis by the angle Beta , said element having a second polished surface normal to the incident optical axis; and wherein the second prism element has a third polished surface normal to the optical axis of the beam reflected by its second and first surfaces, whilst the third prism element cooperates with a total reflector for the beam reflected from the second partial reflector; the beam emerging through a polished surface normal to the optical axis of the beam reflected by the second partial reflector and said total reflector, the arrangement being such that: (Sin 1 (1/n) - Sin 1 (1/2nf)) > lambda > (Sin 1 (1/n) + Sin 1 (1/2nf) - 2 Alpha ) (Sin 1 (1/n) + Sin 1 (1/2nf)) > Alpha > (Sin 1 (1/2nf)) Sin 1 (1/n) + Sin 1 (1/2nf) > Beta > Alpha ; where f is the f number or relative aperture of the camera objective, and n is the refractive index of the prism glass material.
 7. A colour television camera optical system as claimed in claim 6, in which (Sin 1 (1/n) - Sin 1 (1/2nf)) > lambda > (Sin 1 (1/n) + Sin 1 (1/2nf) - 2 Alpha )1) Sin 1 (1/n)/2 > Alpha > Sin 1 (1/2nf), (2) Sin 1 (1/n) > Beta > Alpha . 3) 